1. Field of the Invention
The present invention relates in general to a power converter. In particular, the present invention relates to a DC-to-DC converter with high efficiency and power density.
2. Description of the Related Art
FIG. 1 shows a resonant reset dual switch forward DC-to-DC converter. In the converter 10, a switch Q101 turns on to divide the DC input voltage into square wave voltage according to a predetermined duty ratio. The square wave voltage is then transformed to secondary winding of the transformer T101, and is rectified into a DC voltage to output to load by a rectifying device composed of an inductor L101 and a capacitor C101. The converter needs a large input rectifying device and an output rectifying device because the converter has a non-continuous input current and a large output voltage ripple.
Consequently, the size and cost of the converter may increase. In addition, the transformer T101 is reset by the inductor L101 and the capacitor C101 such that the voltage stress of the switch Q101 is very high, and also has high electromagnetic interference (EMI) because the converter Q101 usually operates in a hard switching condition.
FIG. 2 shows a DC-to-DC converter 20 with a synchronous rectifying resonant reset forward circuit. The converter 10 shown in FIG. 1 may have a low power efficiency due to consumption of diodes D101 and D102 when the converter 10 is applied to low input voltage, for example below 12 volts. As shown in FIG. 2, the diodes D101 and D102 of the converter 20 are replaced by synchronous rectifiers Q102 and Q103 to increase power efficiency. The synchronous rectifiers Q102 and Q103 are driven by the induced voltage across the secondary winding N102 of the transformer T101.
FIG. 3 illustrates the key operating waveform of the converter 20 shown in FIG. 2. As shown in FIG. 3, the first synchronous rectifier Q102 is always turned on when the switch Q101 is turned on (t1 to t2), and the synchronous effect is ideal. The second synchronous rectifier Q103, however, is not turned on when the switch Q101 is turned off (t3 to t4), thus the synchronous effect is less than ideal. This problem may affect whole efficiency of the converter 20 when the converter is applied to low output voltage. Although the converter 20 has a simple structure, the synchronous effect is not ideal, and the input current ripple and the output voltage ripple are high. Thus, the converter 20 also needs a large rectifier.
It is an object of the invention to provide a novel and improved DC-to-DC converter that can overcome the above-mentioned disadvantages of the related art.
In the present invention, the converter has three inductors, two capacitors, a first switch and a second switch, a first rectifier and a second rectifier, and a transformer with a primary winding and a secondary winding.
The first switch and the second switch are turned on alternately according to a controlling signal, and a current may flow through the primary winding of the transformer thereby transferring energy to the second winding. The first rectifier and the second rectifier operate according to the energy transferred from the primary winding to obtain a solid current through the third inductor, and a solid DC output voltage is output to the load.